1. Field of the Invention
The invention in general relates to the fabrication of integrated circuits, and more particularly to the formation of liquid precursor solutions and the use of the liquid precursors in the fabrication of metal oxide thin films which are included in integrated circuits.
2. Statement of the Problem
Metal oxide thin films are known to be useful in making integrated circuits. For example, barium strontium titanate, commonly referred to as BST, is known to be useful in making integrated circuit thin film capacitors having high dielectric constants. See for example, Kuniaki Koyama, et al., "A Stacked Capacitor With (Ba.sub.-x Sr.sub.x)TiO.sub.3 For 256M DRAM" in IDEM(International Electron Devices Meeting) Technical Digest, December 1991, pp. 32.1.1-32.1.4, and U.S. Pat. No. 5,122,923 issued to Shogo Matsubara et al. Both these references use the conventional process of depositing BST in integrated circuits, i.e. sputtering. Sputtering is inherently hard to control. While useful in conventional silicon technology, when used in combination with capacitors that use metal oxides as the dielectric, sputtering results in capacitors that have relatively high leakage current, fatigue significantly, and generally have undesirable electrical properties.
Liquid precursors have also been used for making certain metal oxides. See G. M. Vest and S. Singaram, "Synthesis of "Metallo-organic Compounds For MOD Powders and Films", Materials Research Society Symposium Proceedings, Vol. 60, 1986, pp. 35-42, Robert W. Vest and Jiejie Xu, "PbTiO.sub.3 Thin Films From Metalloorganic Precursors", IEEE Transactions On Ultrasonics, Ferroelectrics, and Frequency Control, Vol 35, No. 6, Nov. 1988, pp. 711-717, "Metalorganic Deposition (MOD): A Nonvacuum, Spin-on, Liquid-Based, Thin Film Method", Materials Research Society Bulletin., October 1989, pp. 48-53, and W. W. Davidson, S. G. Shyu, R. D. Roseman, and R. C. Buchanan, "Metal Oxide Films From Carboxylate Precursors", in Materials Research Society Symposium Proceedings, Vol. 121, pp. 797-802, 1988. In this disclosure the term "liquid" does not include gels produced in sol-gel processes such as those disclosed in B. M. Melnick, J. D. Cuchiaro, L. D. McMillan, C. A. Paz De Araujo, and J. F. Scott, "Process Optimization and Characterization of Device Worthy Sol-Gel Based PZT For Ferroelectric Memories," in Ferroelectric, Vol. 109, pp. 1-23, 1990, and U.S. Pat. No. 4,946,710 issued to William D. Miller, Leo N. Chapin, and Joseph T. Evans.
In the prior art processes employing liquid precursors, a metal-organic compound is dissolved in a suitable liquid solvent which is applied to a substrate, usually by spin-coating. In the Vest references (the first three in the group just cited) and the Davidson reference, the preferred solvent is xylenes although toluene and benzene are also mentioned. However, the metal-organic compound heated to high temperatures and dried to a solid or semisolid before dissolving it in xylene. While some prior art references speculate that the liquid deposition process might have advantages over sputtering or the sol-gel process in the fabrication of electronic devices, the parameters of the actual devices made by the authors and discussed in the references were not in the range in which useful electronic devices could be produced. For example, in the Vest and Singram reference, the grain size reported was 2000 .ANG. minimum, which is as large or larger than the film thickness typically used for capacitor dielectrics in integrated circuits, and in the Vest and Xu reference, the hysteresis loops obtained were not suitable for useful electronic devices. Thus, up to the time of the present invention, the liquid precursor processes have not been useful to make commercial electronic devices.
3. Solution to the problem
The present invention solves the above problems by providing a liquid precursor solution comprising an oxygenated metallic moiety dissolved in a first solvent, then, prior to applying the precursor to an integrated circuit substrate, the first solvent is exchanged for a second solvent to produce a final liquid precursor solution. The precursor remains a liquid throughout the process. The first solvent is preferably a solvent in which the oxygenated metallic moiety dissolves readily and preferably results in a precursor which remains essentially unchanged during storage. The second solvent is preferably one that is adheres well to the substrate, i.e. wets the substrate well, and which disassociates from the metal oxide in subsequent heating steps in a manner that leaves a smooth, dense uniform coating without cracks and other imperfections.
Preferably the second solvent has a higher boiling point than the first solvent. This not only facilitates solvent exchange, but results in slower dissociation which results in less cracking and imperfections. However, it is possible for the second solvent to be a mixture of solvents, including a solvent or solvents that have a lower boiling point than the first solvent, provided one of the solvents has a higher boiling point.
In an exemplary embodiment, capacitors made with a dielectric material comprising barium strontium titanate (BST) thin films having the formula Ba.sub.1-x Sr.sub.x TiO.sub.3, where x is 0.03, were found to have a dielectric constant of nearly 500 and a leakage current of about 10.sup.-9 amps/cm.sup.2 when made by the process of the invention. The above dielectric constant is about twice as large and the leakage current is about ten times as small as the best respective properties reported in the prior art for BST thin films.
The invention provides a method of fabricating a metal oxide thin film for use in fabricating an integrated circuit comprising the steps of: providing an integrated circuit substrate; providing an initial liquid precursor comprising a metallic compound in a first solvent; adding a second solvent to the initial liquid precursor to form an intermediate precursor; heating the liquid precursor to distill off the first solvent and form a final liquid precursor that is capable of forming a metal oxide thin film having better electrical properties than the thin films formed by the initial precursor; applying the final liquid precursor to the substrate; and treating the final precursor on the substrate to form a thin film of the metal oxide. Preferably, the first solvent is a liquid that reacts with the metal and the second solvent comprises a liquid that does not react with the metal. Preferably, the final precursor liquid wets the substrate better than the first precursor liquid. Preferably, the metallic compound comprises a compound selected from the group consisting of metal carboxylates, metal alkoxides, and metal alkoxycarboxylates, and the metal oxide comprises one or more compounds selected from the group consisting of barium strontium titanate, strontium titanate, barium titanate, bismuth titanate, strontium bismuth tantalate, strontium bismuth niobate, strontium bismuth tantalum niobate, barium bismuth tantalate, lead bismuth tantalate, and barium bismuth niobate. Preferably, the second solvent comprises a liquid selected from the group consisting of xylenes and n-butyl acetate. Preferably, the first solvent comprises 2-methoxyethanol and the second solvent comprises xylene. Preferably, the step of applying comprises spin-coating.
In another aspect, the invention provides a method of fabricating a metal oxide thin film for use in an integrated circuit comprising the steps of: providing an integrated circuit substrate; providing an initial liquid precursor comprising a metallic compound dissolved in a first solvent and a second solvent; heating the initial liquid precursor to distill off the first solvent and form a final liquid precursor that is capable of forming a metal oxide thin film having better electrical properties than the thin films formed by a precursor including the first solvent; applying the final liquid precursor to the substrate; and treating the final precursor on the substrate to form a thin film of the metal oxide.
In a further aspect, the invention provides a method of fabricating a metal oxide thin film for use in an integrated circuit, the method comprising the steps of: providing an integrated circuit substrate and a liquid precursor comprising a metallic compound dissolved in an alcohol and a second solvent; heating the liquid precursor to distill off the alcohol and form a final liquid precursor comprising the metallic compound dissolved in the second solvent; applying the precursor to the substrate; and treating the precursor on the substrate to evaporate the second solvent and form a thin film of the metal oxide. Preferably, the step of providing a liquid precursor comprises the steps of: reacting a metal or a metallic compound with the first solvent to form an initial liquid precursor comprising a reactant dissolved in the first solvent; and adding the second solvent to the initial liquid precursor. Preferably, the alcohol is selected from 2-methoxyethanol and isopropanol. Preferably, the alcohol is 2-methoxyethanol and the second solvent comprises xylene, n-butyl acetate, or a combination thereof. Preferably, the step of applying comprises spin-coating and the step of treating comprises drying and annealing the final precursor on the substrate. Preferably, the metallic compound comprises a compound selected from the group consisting of metal carboxylates, metal alkoxides, and metal alkoxycarboxylates. Preferably, the metal oxide comprises one or more compounds selected from the group consisting of barium strontium titanate, strontium titanate, barium titanate, bismuth titanate, strontium bismuth tantalate, strontium bismuth niobate, strontium bismuth tantalum niobate, barium bismuth tantalate, lead bismuth tantalate, and barium bismuth niobate.
In another aspect the invention provides a method of fabricating a metal oxide thin film for use in an integrated circuit comprising the steps of: providing an integrated circuit substrate; providing an initial liquid precursor comprising a metallic compound in a first solvent; storing said initial liquid precursor longer than one week; adding a second solvent to the liquid precursor; heating the liquid precursor to distill off the first solvent and form a final precursor that is capable of forming a metal oxide thin film having better electrical properties than the thin films formed by the initial precursor; applying the final precursor to the substrate; and treating the final precursor on the substrate to form a thin film of the metal oxide.
In another aspect the invention provides a method of making a final liquid precursor for use in manufacturing metal oxide thin films for use in integrated circuits, the method comprising the steps of: providing an initial liquid precursor comprising an oxygenated metallic moiety dissolved in a first solvent; and performing a solvent exchange step on the initial liquid precursor to form the final precursor. Preferably, the solvent exchange step comprises adding a second solvent to the liquid initial precursor and distilling off the first solvent. Preferably, the first solvent comprises a liquid selected from the group comprising carboxylic acids and alcohols and the second solvent comprises a solvent selected from the group consisting of xylenes and n-butyl acetate. Preferably, the oxygenated metallic moiety comprises a compound selected from the group consisting of metal carboxylates, metal alkoxides, and metal alkoxycarboxylates. Preferably, the metal oxide comprises one or more compounds selected from the group consisting of barium strontium titanate, strontium titanate, barium titanate, bismuth titanate, strontium bismuth tantalate, strontium bismuth niobate, strontium bismuth tantalum niobate, barium bismuth tantalate, lead bismuth tantalate, and barium bismuth niobate.
In still another aspect, the invention provides a method of making a final liquid precursor for use in manufacturing metal oxide thin films for use in integrated circuits, the method comprising the steps of: providing an initial liquid precursor comprising an oxygenated metallic moiety dissolved in a solvent; and performing a xylene exchange step on the initial liquid precursor to form the final precursor.
The invention also provides a method of fabricating a metal oxide thin film containing bismuth for use in an integrated circuit, the method comprising the steps of: providing an integrated circuit substrate; providing a metal compound solution comprising an oxygenated metallic moiety dissolved in a solvent; providing a bismuth solution comprising a bismuth compound in a solvent; and adding the bismuth solution to the metal compound solution to form the liquid precursor and applying the liquid precursor to the substrate, all the while maintaining the bismuth solution and the liquid precursor at a temperature of 30.degree. C. or less; and treating the final precursor on the substrate to form a thin film of the metal oxide. Preferably the metal oxide is a compound selected from the group consisting of strontium bismuth tantalate, strontium bismuth niobate, and strontium bismuth tantalum niobate.
In another aspect the invention provides a method of making a liquid precursor for use in manufacturing metal oxide thin films containing bismuth for use in integrated circuits, the method comprising the steps of: providing a metal compound solution at a temperature of greater than 30.degree. C., the metal compound solution comprising an oxygenated metallic moiety dissolved in a solvent; cooling the metal compound solution to 30.degree. C. or below; providing a bismuth solution comprising a bismuth compound in a solvent; and adding the bismuth solution to the cooled metal compound solution to form the precursor. Preferably, the metallic moiety includes a metal selected from the group consisting of strontium, tantalum, and niobium.
The precursors of the invention not only form thin films with better electrical properties, but are less sensitive to handling and easier to use, and thus lend themselves more readily to mass production of integrated circuits. Numerous other features, objects and advantages of the invention will become apparent from the following description when read in conjunction with the accompanying drawings.